Hot press molding and manufacturing method therefor

ABSTRACT

A hot press molding including: a first forming region exhibiting a metal structure, which contains 80-97 area % of martensite and 3-20 area % of retained austenite, respectively, and in which the residual structure is 5 area % or less; and a second forming region exhibiting a metal structure, which contains 70-97 area % of bainitic ferrite, 27 area % or less of martensite, and 3-20 area % of retained austenite, respectively, and in which the residual structure is 5 area % or less. As a result, hot press moldings, which have at least a region corresponding to a shock-resistant area and a region corresponding to an energy-absorbing area in a single molding and in which a high level of balancing of high strength with elongation according to the respective region can be achieved, are provided without using a welding method.

TECHNICAL FIELD

The present invention relates to a hot press molded article which isused for a structural member of an automobile component and whosestrength and ductility can be adjusted according to different regions inthe molded article and to the method for producing such a hot pressmolded article. More specifically, the present invention relates to ahot press molded article exhibiting strength and ductility according todifferent regions in such a manner that when a pre-heated steel plate(blank) is molded into a predetermined shape, thermal treatment isapplied at the same time as shaping, and also relates to the usefulmethod for producing such a hot press molded article.

BACKGROUND ART

Development has been made to reduce the weight of vehicle bodies as oneof the measures for improvement of the fuel efficiency of automobiles,such a measure stemming from global environmental issues. A highestpossible strength has been required for steel plates used for theautomobiles. However, with an increase in the strength of the steelplates for the purpose of reducing the weight of the automobiles, anelongation EL (elongation) and an r value (Lankford value) decrease,resulting in a lower press formability and a lower shape fixability.

In order to solve the above-described problem, the following hot pressmolding method has been employed for component production. After a steelplate is heated to a predetermined temperature (e.g., a temperature atwhich an austenite phase is exhibited) to lower strength (i.e., tofacilitate molding), molding is, for shaping, performed using a diehaving a lower temperature (e.g., a room temperature) than that of thethin steel plate. At the same time, rapid-cooling thermal treatment(quenching) is performed using a temperature difference between the dieand the steel plate, thereby ensuring strength after molding.

According to the above-described hot press molding method, since moldingis performed in a low-strength state, springback is small (favorableshape fixability is obtained). In addition, since a material containing,e.g., alloy elements of Mn and B and exhibiting favorable hardenabilityis used, a 1500 MPa class strength in terms of tensile strength can beobtained by rapid cooling. Note that the above-described hot pressmolding method is, in addition to hot pressing, called as various namessuch as hot forming, hot stamping, a hot stamp method, and diequenching.

FIG. 1 is a schematic view illustrating a die structure for performinghot press molding (hereinafter sometimes represented by “hot stamping”)as described above. In FIG. 1, a reference numeral “1” denotes a punch,a reference numeral “2” denotes a die, a reference numeral “3” denotes ablank holder, a reference numeral “4” denotes a steel plate (blank),reference characters “BHF” denote wrinkle pressing force, referencecharacters “rp” denote a punch shoulder radius, reference characters“rd” denote a die shoulder radius, and reference characters “CL” denotea clearance between the punch and the die. Of these components, thepunch 1 and the die 2 are formed respectively therein passages 1 a, 2 athrough which a corresponding one of cooling media 5 a, 6 a (e.g.,water) is able to pass. Configuration is made such that the coolingmedia 5 a, 6 a pass through these passages to cool the punch 1 and thedie 2.

In hot stamping (e.g., hot deep drawing) using the above-described die,molding begins in such a state that the steel plate (blank) 4 is heatedto a single-phase temperature range of an Acs transformation point orhigher and then, is softened. That is, in the state in which the steelplate 4 in a high-temperature state is interposed between each pair ofthe die 2 and the blank holder 3, the punch 1 pushes the steel plate 4(in the direction indicated by an arrow A) into a hole of the dies 2(the space between the dies 2, 2 in FIG. 1). While the outer diameter ofthe steel plate 4 is being reduced, the steel plate 4 is molded into ashape corresponding to the outer shape of the punch 1. The punch 1 andthe dies 2 are cooled in parallel with molding to draw heat from thesteel plate 4 to the die (the punch 1 and the dies 2), and are furtherheld and cooled at a lower dead point in molding (the point at which apunch tip end is positioned innermost: the state illustrated in FIG. 1)to quench the raw material (the steel plate 4). Such a molding methodcan be performed to obtain a 1500 MPa class molded article withfavorable dimension accuracy. Moreover, since a forming load can bereduced as compared to the case of molding a component of the samestrength class as the above-described molded article in a cold state,the capacity of a pressing machine can be smaller.

Steel plates containing 22MnB5 steel as a raw material have been knownas widely-used current steel plates for hot stamping. These steel plateshave a tensile strength of 1500 MPa and an elongation of about 6-8%, andhave been applied to shock-resistant members (members least deformingand rupturing in collision). Moreover, further development has been madeto increase a C content and increase strength (1500 MPa or higher, a1800 MPa class) with 22MnB5 steel being used as a base material.

However, in current situation, steel grades other than 22MnB5 steel havebeen little applied, and little study has been made on steel grades andmethods in order to control the strength and elongation of a component(e.g., strength reduction: a 980 MPa class, elongation enhancement: 20%)to expand the range of application beyond application to theshock-resistant members.

In passenger cars of a medium size or larger, components such as Bpillars (center pillars), rear side members, and front side members maysometimes have both functions of a shock-resistant area and anenergy-absorbing area, considering compatibility (the function of alsoprotecting a collision partner in collision with a small-sized vehicle)in lateral collision or rear collision. In order to produce thesemembers, the following method has been mainly employed: super-hightensile steel having a high strength of a 980 MPa class and high tensilesteel having an elongation of a 440 MPa class are, for example,laser-welded (form a tailor welded blank (TWB)) to press-mold the TWB ina cold state. However, in recent years, development has been made on thetechnique of forming, by hot stamping, a component with portionsdifferent from each other in strength.

For example, Non-Patent Document 1 proposes the method for laser-welding(forming a tailor welded blank (TWB)) 22MnB5 steel for hot stamping anda material whose strength is not increased by quenching in a die andthen, performing hot stamping. Different portions are formed in acomponent such that a tensile strength is 1500 MPa (an elongation of6-8%) on a high-strength side (a shock-resistant area side) and that thetensile strength is 440 MPa (an elongation of 12% or higher) on alow-strength side (an energy-absorbing area side). From a similar pointof view, the technique as described in Non-Patent Document 2 has beenalso proposed.

In the techniques of Non-Patent Documents 1 and 2, on theenergy-absorbing area side, the tensile strength is 600 MPa or lower,and the elongation is about 12-18%. It is required to performlaser-welding (form the tailor welded blank (TWB)) in advance, leadingto an increase in the number of steps and an increase in a cost.Moreover, an energy-absorbing area which intrinsically does not need tobe quenched is heated, and therefore, these techniques are notpreferable considering heat consumption.

In addition, e.g., the techniques as described in Non-Patent Documents 3and 4 have been also proposed as the technique of forming portionsdifferent from each other in strength in a component. In the techniquedescribed in Non-Patent Document 3, the portions different from eachother in strength are formed in such a manner that a temperaturedifference (distribution) is given to a blank in a heating furnace.Since 22MnB5 steel is used as a base material, poor robustness of thepost-quenching strength against heating in a two-phase temperature rangeis exhibited due to addition of boron. Moreover, it is difficult tocontrol the strength on the energy-absorbing area side, and only anelongation of about 15% can be obtained.

On the other hand, in the technique described in Non-Patent Document 4,the portions different from each other in strength are formed in such amanner that a cooling rate is changed in a die (by heating part of thedie with a heater or using materials different from each other incoefficient of thermal conductivity). Since 22MnB5 steel is used as abase material, it is not rational in terms of controlling (die coolingcontrol) such that 22MnB5 steel intrinsically having favorablehardenability is not quenched.

CITATION LIST Non-Patent Document

-   Non-Patent Document 1: Klaus Lamprecht, Gunter Deinzer, Anton Stich,    Jurgen Lechler, Thomas Stohr, Marion Merklein, “Thermo-Mechanical    Properties of Tailor Welded Blanks in Hot Sheet Metal Forming    Processes,” Proc. IDDRG2010, 2010.-   Non-Patent Document 2: Usibor1500P(22MnB5)/1500    MPa·8%-Ductibor500/550˜700 MPa·17% [searched on Apr. 27, 2011],    Internet    <http://www.arcelomittal.com/tailoredblanks/pre/seifware.pl>-   Non-Patent Document 3: 22MnB5/above AC3/1500 MPa·8%-below    AC3/Hv190·Ferrite/Cementite Rudiger Erhardt and Johannes Boke,    “Industrial application of hot forming process simulation,” Proc, of    1st Int. Conf. on Hot Sheet Metal Forming of High-Performance steel,    ed. By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp    83-88, 2008.-   Non-Patent Document 4: Begona Casas, David Latre, Noemi Rodriguez,    and Isaac Valls, “Tailor made tool materials for the present and    upcoming tooling solutions in hot sheet metal forming,” Proc, of 1st    Int. Conf. on Hot Sheet Metal Forming of High-Performance steel, ed.    By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp    23-35, 2008.

SUMMARY OF THE INVENTION Technical Problems

The present invention has been made in view of the foregoing situation,and is intended to provide hot press molded articles having, withoutapplication of welding, at least regions corresponding respectively to ashock-resistant area and an energy-absorbing area in a single moldedarticle and exhibiting a high-level balance between high strength andelongation according to each region and to provide the useful method forproducing such a hot press molded article.

Solution to Problems

The hot press molded article of the present invention capable ofaccomplishing the above-described objective is a hot press moldedarticle formed by hot press molding of a thin steel plate, whichincludes a first molding region exhibiting a metal structure whichcontains 80-97 area % of martensite and 3-20 area % of retainedaustenite and which has a residual structure at 5 area % or less; and asecond molding region exhibiting a metal structure which contains 70-97area % of bainitic ferrite, 27 area % or less of martensite, and 3-20area % of retained austenite and which has a residual structure at 5area % or less.

In the hot press molded article of the present invention, the chemicalcomponent composition thereof is not limited. However, examples of thechemical component composition include a chemical component compositionin which the first and second molding regions have an identical chemicalcomponent composition, and steel of each component region contains, inunits of mass %, 0.15-0.3% of C, 0.5-3% of Si, 0.5-2% of Mn, 0.05% orless of P, 0.05% or less of S, 0.01-0.1% of Al, 0.01-1% of Cr,0.0002-0.01% of B, [N]×4-0.1% of Ti, and 0.001-0.01% of N, where 0% isnot inclusive for the P and the S and [N] denotes an N content in unitsof %, and the steel of each component region has a residual consistingof iron and an inevitable impurity.

In the hot press molded article of the present invention, it is usefulthat as necessary, the steel further contains, as other element, (a) oneor more selected from a group consisting of Cu, Ni, and Mo in a totalamount of 1% or less, where 0% is not inclusive, and that the steelfurther contains, as other element, (b) at least one of V or Nb in atotal amount of 0.1% or less, where 0% is not inclusive. Depending onthe types of elements to be contained, the characteristics of the hotpress molded article are further improved.

The method of the present invention is the method for producing theabove-described hot press molded article by forming a thin steel plateso as to divide the thin steel plate into a plurality of regionsincluding at least first and second molding regions, which includesafter the thin steel plate is heated to a temperature of an Acstransformation point or higher and 1000° C. or lower, starting coolingat an average cooling rate of 20° C./sec or higher and molding bypressing at least the first and second molding regions together with adie; and terminating, in the first molding region, the molding at equalto or lower than a temperature lower than a martensite transformationstart temperature by 50° C., and performing, in the second moldingregion, the cooling to a temperature range of equal to or lower than atemperature lower than a bainite transformation start temperature by100° C. and equal to or higher than the martensite transformation starttemperature and terminating the molding after a lapse of a stay time of10 seconds or longer within the temperature range.

Effects of the Invention

According to the present invention, in the hot press molding method, theconditions therefor are properly controlled according to each region ofthe molded article. This can adjust the metal structure of each regionwhile a proper amount of retained austenite is present. Moreover, thehot press molded article can be achieved, which has a higher intrinsicductility (residual ductility) as compared to a conventional case ofusing 22MnB5 steel. Further, combination of thermal treatment conditionsand a pre-molded steel plate can properly control the strength and theelongation according to each region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a die configuration forperforming hot press molding.

FIG. 2 is a schematic view illustrating a die used in an example.

FIGS. 3( a) and 3(b) are schematic views illustrating the shape of apress molded article formed in the example.

DESCRIPTION OF EMBODIMENTS

The inventor(s) of the present invention has conducted study fromvarious angles in order to realize a hot press molded article which whena thin steel plate is heated to a predetermined temperature to produce amolded article by hot press molding, exhibits strength corresponding tothe demand characteristics of each of different regions and exhibitsfavorable ductility (elongation) after molding.

As a result, the present invention has been accomplished based on thefollowing findings. When a thin steel plate is press-molded using a diefor press molding to produce a hot press molded article, a heatingtemperature and the conditions for each molding region in molding areproperly controlled, and the structure of each molding region isadjusted so as to contain 3-20 area % of retained austenite. Thisrealizes a hot press molded article exhibiting a strength-elongationbalance according to each molding region.

The reasons for setting the range of each structure (a basic structure)in each molding region of the hot press molded article of the presentinvention are as follows.

(Structures of First Molding Region)

High-strength martensite is used for a main structure of a first moldingregion, thereby ensuring a high strength in a particular region of thehot press molded article. From this point of view, it is necessary thatthe area fraction of the martensite is 80 area % or greater. However,when such a fraction exceeds 97 area %, the area fraction of retainedaustenite (a retained austenite fraction) is insufficient, and ductility(residual ductility) is lowered. The lower limit of the martensitefraction is preferably 83 area % or greater (more preferably 85 area %or greater), and the upper limit of the martensite fraction ispreferably 95 area % or less (more preferably 93 area % or less).

The retained austenite has the effect of transforming into themartensite during plastic deformation to increase a work hardening rate(transformation-induced plasticity) and improving the ductility of themolded article. In order to produce such an effect, it is necessary thatthe retained austenite fraction is 3 area % or greater. A greaterretained austenite fraction results in better ductility. However, in acomposition used for steel plates for automobiles, the retainedaustenite which can be ensured is limited, and the upper limit thereofis about 20 area %. The lower limit of the retained austenite fractionis preferably 5 area % or greater (more preferably 7 area % or greater).

In addition to the above-described structures, ferrite, pearlite,bainite, etc. may be contained as a residual structure. These structuresare softer than the martensite, and less contribute to the strength ascompared to other structures. For this reason, these structures arepreferably contained in the minimum possible amount. Note that thesestructures can be contained up to 5 area %. The residual structure ismore preferably 3 area % or less, and much more preferably 0 area %.

Since the structures of the first molding region are formed as describedabove, a portion (e.g., a shock-resistant area of an automobilecomponent) where the strength (a tensile strength TS) is 1500 MPa orgreater and the elongation (a total elongation EL) is 10% or greater canbe formed.

(Structures of Second Molding Region)

Since high-strength bainitic ferrite having sufficient ductility is usedfor a main structure of a second molding region, both of a high strengthand a high ductility of the hot press molded article can be realized.From this point of view, it is necessary that the area fraction of thebainitic ferrite (a bainitic ferrite fraction) is 70 area % or greater.However, when such a fraction exceeds 97 area %, the retained austenitefraction is insufficient, and the ductility (the residual ductility) islowered. The lower limit of the bainitic ferrite fraction is preferably75 area % or greater (more preferably 80 area % or greater), and theupper limit of the bainitic ferrite fraction is preferably 95 area % orless (more preferably 90 area % or less).

The high-strength martensite can be partially contained to increase thestrength of the hot press molded article. However, a greater amount ofmartensite results in a lower ductility (a lower residual ductility).From this point of view, it is necessary that the area fraction of themartensite (the martensite fraction) is 27 area % or less. The lowerlimit of the martensite fraction is preferably 5 area % or greater (morepreferably 10 area % or greater), and the upper limit of the martensitefraction is preferably 20 area % or less (more preferably 15 area % orless).

Because of the reasons similar to those of the first molding region, theretained austenite fraction is 3 area % or greater and 20 area % orless. The preferable lower limit of the retained austenite fraction issimilar to that of the first molding region.

In addition to the above-described structures, ferrite, pearlite,bainite, etc. may be contained as a residual structure. These structuresare softer than the martensite, and less contribute to the strength ascompared to other structures. For this reason, these structures arepreferably contained in the minimum possible amount. Note that thesestructures can be contained up to 5 area %. The residual structure ismore preferably 3 area % or less, and much more preferably 0 area %.

Since the structures of the second molding region are formed asdescribed above, a portion (e.g., an energy-absorbing area of theautomobile component) where the strength (the tensile strength TS) is1100 MPa or greater and the elongation (the total elongation EL) is 15%or greater can be formed.

The molded article of the present invention includes at least the firstand second molding regions, but does not necessarily include only twomolding regions. The molded article of the present invention may furtherinclude a third or fourth molding region. These molding regions can beformed according to a production method described later.

In production of the hot press molded article of the present invention,a thin steel plate (having the same chemical component composition asthat of the molded article) may be formed so as to be divided into aplurality of regions including at least the first and second moldingregions. Specifically, the above-described thin steel plate may beheated to a temperature of an Acs transformation point or higher and1000° C. or lower. Then, for at least the first and second moldingregions, cooling at an average cooling rate of 20° C./sec or higher andmolding may begin by pressing of the first and second molding regionstogether with the die. In the first molding region, molding may beterminated at equal to or lower than a temperature (hereinaftersometimes referred to as “Ms point—50° C.”) lower than a martensitetransformation start temperature by 50° C. In the second molding region,cooling may be performed to a temperature range of equal to or lowerthan a temperature (hereinafter sometimes referred to as “Bs point—100°C.”) lower than a bainite transformation start temperature by 100° C.and equal to or higher than the martensite transformation starttemperature (the Ms point), and molding may be terminated after a lapseof a stay time of 10 seconds or longer within the above-describedtemperature range. The reasons for setting each requirement in thismethod are as follows. Note that the phrase of “molding is terminated”basically means the state at a lower dead point in molding (the point atwhich a punch tip end is positioned innermost: the state illustrated inFIG. 1). However, when the die needs to be cooled to a predeterminedtemperature in such a state, the phrase of “molding is terminated” alsomeans the state until the die is detached after the die is maintainedcooled.

According to the above-described method, the steel plate is divided intoat least two molding regions (e.g., a high-strength region and alow-strength region), and the production conditions are controlledaccording to each region. Thus, the molded article exhibiting thestrength-ductility balance according to each region can be obtained. Theproduction conditions for forming each region will be described.

(Production Conditions for First Molding Region (High-Strength Region))

In order to properly adjust the structures of the hot press moldedarticle, it is necessary to control the heating temperature within apredetermined range. By proper control of the heating temperature, apredetermined amount of retained austenite can be ensured at asubsequent cooling step. Meanwhile, the first molding region cantransform into the structure mainly containing the martensite. As aresult, the final hot press molded article can be formed with desiredstructures. If the heating temperature of the thin steel plate is lowerthan the Ac₃ transformation point, a sufficient amount of austenitecannot be obtained in heating, and a predetermined amount of retainedaustenite cannot be ensured at the final structure (the structures ofthe molded article). On the other hand, if the heating temperature ofthe thin steel plate exceeds 1000° C., the particle size of theaustenite increases in heating, and the martensite transformation starttemperature (the Ms point) and a martensite transformation endtemperature (an Mf point) increase. Thus, the retained austenite cannotbe ensured in quenching, and as a result, favorable moldability cannotbe achieved. The heating temperature is preferably (Acs transformationpoint+50° C.) or higher and 950° C. or lower.

The cooling conditions in molding and a molding end temperature need tobe properly controlled according to each region. First, in a steel plateregion (hereinafter sometimes referred to as a “first steel plateregion”) corresponding to the first molding region of the moldedarticle, it is necessary that an average cooling rate of 20° C./sec orhigher is ensured at the die in molding and that molding is terminatedat a temperature of (Ms point—50° C.) or lower.

In order that the austenite formed at the above-described heating stephas a desired structure (a structure mainly containing the martensite)with generation of the structures such as ferrite, pearlite, and bainitebeing blocked, the average cooling rate in molding and the molding endtemperature need to be properly controlled. From this point of view, theaverage cooling rate in molding is 20° C./sec or higher, and the moldingend temperature is (Ms point—50° C.) or lower. In particular, in thecase where a steel plate with a high Si content is targeted, the mixedstructure of the martensite and the retained austenite can be formed bycooling under the above-described conditions. The average cooling ratein molding is preferably 30° C./sec or higher (more preferably 40°C./sec or higher).

While the molding end temperature in the first steel plate region iscooled to a room temperature at the above-described average coolingrate, molding may be terminated. However, after the molding endtemperature is cooled to (Ms point—50° C.) or lower (preferably to atemperature of Ms point—50° C.), cooling (two-step cooling) may beperformed to 200° C. or lower at an average cooling rate of 20° C./secor lower. Such addition of the cooling step allows thickening of carbonof the martensite in untransformed austenite, and therefore, the amountof retained austenite can be increased. In such two-step cooling, theaverage cooling rate in a second cooling step is preferably 10° C./secor lower (more preferably 5° C./sec or lower).

(Production Conditions for Second Molding Region (Low-Strength Region))

On the other hand, in order to properly adjust the structures of thesecond molding region of the hot press molded article, it is necessaryto control the heating temperature of a steel plate region (hereinaftersometimes referred to as a “second steel plate region”) corresponding tothe second molding region within a predetermined range. By propercontrol of the heating temperature, a predetermined amount of retainedaustenite can be ensured at a subsequent cooling step. Meanwhile, thesecond molding region can transform into the structure mainly containingthe bainitic ferrite. As a result, the final hot press molded articlecan be formed with desired structures. If the heating temperature of thethin steel plate is lower than the Acs transformation point, asufficient amount of austenite cannot be obtained in heating, and apredetermined amount of retained austenite cannot be ensured at thefinal structure (the structures of the molded article). On the otherhand, if the heating temperature of the thin steel plate exceeds 1000°C., the state similar to that of the first steel plate region is broughtabout (the preferable temperature range is also similar to that of thefirst steel plate region).

In order that the austenite formed at the above-described heating stephas a desired structure (a structure mainly containing the bainiticferrite) with generation of the structures such as ferrite and pearlitebeing blocked, the average cooling rate in molding and a cooling endtemperature need to be properly controlled. From this point of view, theaverage cooling rate in molding needs to be 20° C./sec or higher, andthe cooling end temperature needs to be (Bs point—100° C.) or lower andthe martensite transformation start temperature (the Ms point) or higher(such a temperature range is hereinafter sometimes referred to as a“cooling rate change temperature”). The average cooling rate ispreferably 30° C./sec or higher (more preferably 40° C./sec or higher).

Cooling is temporarily stopped within the above-described temperaturerange (the cooling rate change temperature), and such a state stays for10 seconds or longer within the above-described temperature range (i.e.,a temperature range of (Bs point—100° C.) or lower and the martensitetransformation start temperature Ms point or higher). In this manner,bainite transformation proceeds in supercooled austenite so that thestructure mainly containing the bainitic ferrite can be formed. A staytime in this state is preferably 50 seconds or longer (more preferably100 seconds or longer). If the stay time is too long, the austenitebegins decomposing, and the retained austenite fraction cannot beensured. For this reason, the stay time is preferably 1000 seconds orshorter (more preferably 800 seconds or shorter).

As long as the stay step as described above is performed within theabove-described temperature range, the stay step may be any of anisothermal holding step, a monotonic cooling step, or a re-heating step.Moreover, regarding the relationship between the above-described staystep and molding, the stay step as described above may be added aftermolding is terminated, or the holding step may be added within theabove-described temperature range in the middle of termination ofmolding. After molding is terminated as described above, cooling may beperformed to the room temperature by cold heat radiation at a propercooling rate.

Control of the average cooling rate in molding can be achieved by a unitsuch as (a) a unit for controlling the temperature of the die (thecooling media illustrated in FIG. 1) and (b) a unit for controlling thecoefficient of thermal conductivity of the die (the same applies tocooling in the later-described method). In the method of the presentinvention, the cooling conditions in molding vary according to eachsteel plate region. The control units such as the units (a) and (b) maybe provided separately in a single die, and cooling control may beperformed corresponding to each steel plate region in the single die.

The method for producing the hot press molded article according to thepresent invention is applicable not only to the case (the direct method)of producing a hot press molded article in a simple shape as illustratedin FIG. 1, but also to the case of producing a molded article in arelatively-complicated shape. Note that in the case of the complicatedcomponent shape, it might be difficult to form the final shape of theproduct by a single process of press molding. In this case, the method(called an “indirect method”) for performing cold press molding as apreceding process of hot press molding can be employed. In this method,a portion which is difficult to be molded is pre-molded into anapproximate shape by cold working, and the other portion ishot-press-molded. According to such a method, e.g., when arecessed-raised portion (ridge portion) is formed at three sections of amolded article, the first and second recessed-raised portions are formedby cold press molding, and then, the third recessed-raised portion isformed by hot press molding.

The present invention is intended for a hot press molded article made ofa high-strength steel plate. Although the steel grade of such a steelplate may include steel grades of high-strength steel plates withtypical chemical component compositions, C, Si, Mn, P, S, Al, Cr, B, Ti,and N are preferably adjusted to suitable ranged. From this point ofview, the preferable ranges of these chemical components and the reasonsfor limiting these ranges are as follows.

(C: 0.15-0.3%)

C is an essential element (the low-strength region) in strengthimprovement made by micronizing the bainitic ferrite generated at thecooling process and increasing a dislocation density in the bainiticferrite. Moreover, C is also an essential element (the high-strengthregion) in control of the strength of the martensite structure. A less Ccontent results in insufficient strength even in full martensite. C isan element heavily involved with hardenability. An increase in the Ccontent produces the effect of reducing formation of other softstructures such as ferrite during cooling after heating. Further, C isalso an element necessary for ensuring the retained austenite. If the Ccontent is less than 0.15%, the bainite transformation start temperatureBs increases, and a high strength of the hot press molded article cannotbe ensured. On the other hand, if the C content becomes excess andexceeds 0.3%, the strength becomes too high, and favorable ductilitycannot be obtained. The lower limit of the C content is more preferably0.18% or greater (much more preferably 0.20% or greater), and the upperlimit of the C content is more preferably 0.27% or less (much morepreferably 0.25% or less).

(Si: 0.5-3%)

Si produces the effect of forming the retained austenite in quenching.Moreover, Si also produces the effect of increasing, by solid solutionstrengthening, the strength without lowering the ductility much. If a Sicontent is less than 0.5%, a predetermined amount of retained autstenitecannot be ensured, and favorable ductility cannot be obtained. On theother hand, if the Si content becomes excess and exceeds 3%, the degreeof solid solution strengthening becomes too high, and the ductility issignificantly lowered. The lower limit of the Si content is morepreferably 1.15% or greater (much more preferably 1.20% or greater), andthe upper limit of the Si content is more preferably 2.7% or less (muchmore preferably 2.5% or less).

(Mn: 0.5-2%)

Mn is a useful element in reduction of formation of ferrite and pearliteduring primary cooling. Moreover, Mn is also a useful element inmicronizing the structure unit of the bainitic ferrite by lowering (Bspoint—100° C.), or in enhancement of the strength of the bainiticferrite by increasing a dislocation density in the bainitic ferrite.Further, Mn is also a useful element in stabilizing the austenite toincrease the amount of retained austenite. In order to produce theseeffects, Mn is preferably contained at 0.5% or greater. Only consideringcharacteristics, a great Mn content is preferable. However, since thecost for alloy addition increases, the Mn content is preferably 2% orless. Moreover, with significant improvement of the strength of theaustenite, a hot rolling load increases, and it is difficult to producea steel plate. For this reason, it is not preferable that the Mn contentexceeds 2%, considering productivity. The lower limit of the Mn contentis more preferably 0.7% or greater (much more preferably 0.9% orgreater), and the upper limit of the Mn content is more preferably 1.8%or less (much more preferably 1.6% or less).

(P: 0.05% or Less (0% is not Inclusive))

Although P is an element inevitably contained in steel, P lowers theductility. For this reason, P is preferably reduced as much as possible.However, significant reduction results in an increase in a steelproduction cost, and in production, it is difficult to make a P content0%. Thus, the P content is preferably 0.05% or less (0% is notinclusive). The upper limit of the P content is more preferably 0.045%or less (much more preferably 0.040% or less).

(S: 0.05% or Less (0% is not Inclusive))

As in P, S is also an element inevitably contained in steel, and lowersthe ductility. For this reason, S is preferably reduced as much aspossible. However, significant reduction results in an increase in thesteel production cost, and in production, it is difficult to make an Scontent 0%. Thus, the S content is preferably 0.05% or less (0% is notinclusive). The upper limit of the S content is more preferably 0.045%or less (much more preferably 0.040% or less).

(Al: 0.01-0.1%)

Al is useful as a deoxidizing element, and is also useful in ductilityimprovement because Al fixes, as AlN, solid liquid N present in steel.In order to effectively produce these effects, an Al content ispreferably 0.01% or greater. However, if the Al content becomes excessand exceeds 0.1%, Al₂O₃ is excessively generated, and the ductility islowered. Note that the lower limit of the Al content is more preferably0.013% or greater (much more preferably 0.015% or greater), and theupper limit of the Al content is more preferably 0.08% or less (muchmore preferably 0.06% or less).

(Cr: 0.01-1%)

Cr has the effect of reducing ferrite transformation and pearlitetransformation. Thus, Cr is an element preventing formation of ferriteand pearlite during cooling and contributing to ensuring the retainedaustenite. In order to produce these effects, Cr is preferably containedat 0.01% or greater. Even if Cr is excessively contained at greater than1%, a cost increases. Moreover, since Cr significantly increases thestrength of the austenite, a hot rolling load increases, and it isdifficult to produce a steel plate. For this reason, it is notpreferable that a Cr content exceeds 1%, considering the productivity.The lower limit of the Cr content is more preferably 0.02% or greater(much more preferably 0.05% or greater), and the upper limit of the Crcontent is more preferably 0.8% or less (much more preferably 0.5% orless).

(B: 0.0002-0.01%)

B has the effect of increasing the hardenability and reducing ferritetransformation and pearlite transformation. Thus, B is an elementpreventing formation of ferrite and pearlite during primary coolingafter heating and contributing to ensuring the bainitic ferrite and theretained austenite. In order to produce these effects, B is preferablycontained at 0.0002% or greater. If B is excessively contained, atgreater than 0.01%, the effect thereof is saturated. The lower limit ofa B content is more preferably 0.0003% or greater (much more preferably0.0005% or greater), and the upper limit of the B content is morepreferably 0.008% or less (much more preferably 0.005% or less).

(Ti: [N]×4-0.1%)

Ti produces the effect of improving the hardenability by fixing N andmaintaining B at a solid solution state. In order to produce such aneffect, Ti is preferably contained at at least equal to or greater thanfour times as great as an N content [N]. However, if a Ti contentbecomes excessive and exceeds 0.1%, a great amount of TiC is formed. Inaddition, the strength increases due to precipitation strengthening, butthe ductility is lowered. The lower limit of the Ti content is morepreferably 0.05% or greater (much more preferably 0.06% or greater), andthe upper limit of the Ti content is more preferably 0.09% or less (muchmore preferably 0.08% or less).

(N: 0.001-0.01%)

Since N is an element capable of reducing a hardenability improvementeffect by fixing B as BN, N is preferably reduced as much as possible.However, reduction of N is limited in an actual process, and for thisreason, the lower limit of the N content is preferably 0.001%. If the Ncontent becomes excessive, coarse TiN particles are formed, and such TiNfunctions as a starting point for destruction to lower the ductility.For this reason, the upper limit of the N content is preferably 0.01%.The upper limit of the N content is more preferably 0.008% or less (muchmore preferably 0.006% or less).

The basic chemical components in the press molded article of the presentinvention are as described above. The residual substantially consists ofiron. Note that the phrase of “substantially consists of iron” canmeans, in addition to iron, not only a slight amount of components(e.g., Mg, Ca, Sr, Ba, REM such as La, and carbide-forming elements suchas Zr, Hf, Ta, W, and Mo) not inhibiting the characteristics of thesteel material of the present invention, but also inevitable impurities(e.g., O and H) other than P and S.

It is useful for the press molded article of the present invention tofurther contain, as necessary, (a) one or more selected from the groupconsisting of Cu, Ni, and Mo in the total amount of 1% or less (0% isnot inclusive) and (b) at least one of V or Nb in the total amount of0.1% or less (0% is not inclusive), for example. Depending on the typesof the elements to be contained, the characteristics of the hot pressmolded article are further improved. The preferable ranges of thesecontained elements and the reasons for limiting these ranges are asfollows.

(One or More Selected from the Group Consisting of Cu, Ni, and Mo in theTotal Amount of 1% or Less (0% is not Inclusive))

Since Cu, Ni, and Mo reduce ferrite transformation and pearlitetransformation, Cu, Ni, and Mo effectively function to prevent formationof ferrite and pearlite during primary cooling and to ensure theretained austenite. In order to produce these effects, Cu, Ni, and Moare preferably contained at the total amount of 0.01% or greater. Onlyconsidering characteristics, a great content is preferable. However,since the cost for alloy addition increases, the total amount ispreferably 1% or less. Moreover, since Cu, Ni, and Mo have the effect ofsignificantly increasing the strength of the austenite, a hot rollingload increases, and it is difficult to produce a steel plate. For thisreason, it is preferable that the total amount is 1% or less,considering the productivity. The lower limit of the total content ofthese elements is more preferably 0.05% or greater (much more preferably0.06% or greater), and the upper limit of the total content of theseelements is more preferably 0.9% or less (much more preferably 0.8% orless).

(At Least One of V or Nb in the Total Amount of 0.1% or Less (0% is notInclusive))

V and Nb have the effect of forming fine carbide particles andmicronizing a structure by a pinning effect. In order to produce theseeffects, at least one of V or Nb is preferably contained at the totalamount of 0.001% or greater. However, if the content of these elementsbecomes excess, coarse carbide particles are formed, this serves as astarting point for destruction to lower the ductility. For this reason,the total content is preferably 0.1% or less. The lower limit of thetotal content of these elements is more preferably 0.005% or greater(much more preferably 0.008% or greater), and the upper limit of thetotal content of these elements is more preferably 0.08% or less (muchmore preferably 0.06% or less).

According to the present invention, the press molding conditions (theheating temperature and the cooling rate according to each steel plateregion) can be properly adjusted to control the characteristics, such asthe strength and the elongation, of the molded article according to eachmolding region. In addition, the hot press molded article can beobtained with a high ductility (a high residual ductility). Thus, thepresent invention is also applicable to a portion (e.g., a memberrequiring both of shock resistance and energy absorption reduction) thathas been difficult to apply in a conventional hot press molded article,and is significantly useful in enlargement of the scope of applicationof the hot press molded article. Further, the molded article of thepresent invention has a higher residual ductility as compared to that ofa molded article whose structure is adjusted by typical annealingperformed after cold press molding.

Advantages of the present invention will be more specifically describedbelow with reference to an example, but the later-described example doesnot limit the scope of the present invention. In light of thedescription made above and later, any design changes may be made withinthe technical scope of the present invention.

This application claims the benefit of and priority to Japanese PatentApplication No. 2013-032615 filed on Feb. 21, 2013, the disclosure ofwhich is hereby incorporated by reference in its entirety in thisapplication.

Example

A steel material having a chemical component composition shown in Table1 below was vacuum-fused, thereby forming an experimental slab. Then,after hot rolling was performed, the slab was cooled and rolled up.Further, cold rolling was performed, thereby forming a thin steel plate.Note that an AC3 transformation point, an Ms point, and (Bs point—100°C.) as shown in Table 1 were obtained using Expressions (1)-(3)described below (e.g., see “The Physical Metallurgy of Steels,” MaruzenCo., Ltd., 1985).

AC₃ Transformation Point (°C.)=910−203×[C]^(1/2)+44.7×[Si]−30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]−11×[Cr]+31.5×[Mo]−20×[Cu]−15.2×[Ni]  (1)

Ms point (°C.)=550−361×[C]−39×[Mn]−10×[Cu]−17×[Ni]−20×[Cr]−5×[Mo]+30×[Al]  (2)

Bs point (° C.)=830−270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−83×[Mo]  (3)

Note that [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu], and[Ni] represent the contents (mass %) of C, Si, Mn, P, Al, Ti, V, Cr, Mo,Cu, and Ni, respectively. If an element(s) shown as each term inExpressions (1)-(3) is not contained, calculation is made without takingsuch an element(s) into consideration.

TABLE 1 Ac₃ Bs Point Steel Chemical Component Composition* (mass %)Transformation Ms Point −100° C. Grade C Si Mn P S Cr Al Ti B N Point (°C.) (° C.) (° C.) A 0.232 1.19 1.41 0.014 0.0021 0.21 0.053 0.027 0.00330.0047 863 409 526 B 0.232 0.18 1.41 0.014 0.0021 0.21 0.053 0.0270.0033 0.0047 817 409 526 *Residual: iron and inevitable impuritiesother than P and S

While the heating temperature was changed in each steel plate region ofthe obtained steel plate, molding and cooling treatment were performed.Specifically, press molding was performed using a bending die having ahat channel shape (a HAT shape) illustrated in FIG. 2. Note that in FIG.2, a reference numeral “10” denotes an upper die (equivalent to thepunch 1 illustrated in FIG. 1), and a reference numeral “11” denotes alower die (equivalent to the die 2 illustrated in FIG. 1). Moreover, inthis die, a pad 12 is provided, and is configured such that pressmolding is performed with a steel plate 4 being interposed between thepad 12 and the upper 11 while pressure (pad pressure) is being applied(at a pad pressure of 9800 N).

The heating temperature and the average cooling rate in each steel plateregion are shown in Table 2 below (the molding end temperature (adie-detaching temperature) was 200° C. in any of the regions). The steelplate size in molding and cooling was 220 mm×500 mm (a plate thicknessof 1.4 mm) (the area ratio between the first steel plate region and thesecond steel plate region was 1:1). The shape of a molded press moldedarticle is illustrated in FIGS. 3( a) and 3(b) (FIG. 3( a) is aperspective view, and FIG. 3( b) is a view schematically illustratingthe cross section). In FIG. 3( a), a reference numeral “15” denotes thefirst steel plate region (corresponding to the first molding region ofthe molded article), and a reference numeral “16” denotes the secondsteel plate region (corresponding to the second molding region of themolded article). Note that “Average Cooling Rate 1” of the first steelplate region as shown in Table 2 is an average cooling rate from theheating temperature to (Ms point—50° C.) or lower (the molding endtemperature), and “Average Cooling Rate 2” of the first steel plateregion is an average cooling rate from the molding end temperature to200° C. or lower.

TABLE 2 Production Conditions Second Steel Plate Region Average FirstSteel Plate Region Cooling Average Steel Plate Average Average RateCooling Rate Cooling Rate Stay Time at Heating Cooling Molding EndCooling (° C./sec) Change (° C./sec) [Bs −100° C. Test Steel TemperatureRate 1 Temperature Rate 2 in Primary Temperature Retention in Secondaryto Ms Point] No. Grade (° C.) (° C./sec) (° C.) (° C./sec) Cooling (°C.) Time (sec) Cooling (sec) 1 A 930 40 200 15 40 N/A N/A N/A 3 2 A 93040 200 15 40 480 0 5 15 3 A 930 40 200 15 40 420 0 20 5 4 A 930 40 20015 40 420 10 50 15 5 A 930 40 200 15 40 600 10 5 23.4 6 B 930 40 200 1540 480 N/A 5 15

For each steel plate subjected to the foregoing processes (heating,molding, and cooling), a tensile strength (TS) and an elongation (atotal elongation EL) were measured in the following manner, and metalstructures (the fraction of each structure) were observed in thefollowing manner.

(Tensile Strength (TS) and Elongation (Total Elongation EL)) A tensiontest was conducted using a JIS 5 test piece, thereby measuring thetensile strength (TS) and the elongation (the total elongation EL). Atthis point, a strain rate in the tension test was 10 mm/sec. In thepresent invention, evaluation was made as “successful” when (a) in thefirst region, the tensile strength (TS) satisfies 1500 MPa or greaterand the elongation (the total elongation EL) satisfies 10% or greaterand (b) in the second region, the tensile strength (TS) satisfies 1100MPa or greater and the elongation (the total elongation EL) satisfies15% or greater.

(Observation of Metal Structures (Fraction of Each Structure))

(1) For the structures of martensite, ferrite, and bainitic ferrite inthe steel plate, the steel plate was corroded with nital, and then, thefraction (the area ratio) of each structure was obtained by scanningelectron microscope (SEM) observation (a magnification of 1000-power or2000-power) with the ferrite and the bainitic ferrite beingdiscriminated from each other.

(2) A retained austenite fraction (an area ratio) in the steel plate wasmeasured in such a manner that after the steel plate was ground to thequarter of the thickness of the steel plate, chemical polishing wasperformed, and then, X-ray diffractometry was performed (e.g., ISJJInt., Vol. 33, 1933, No. 7, P. 776).

(3) The area ratio of the martensite (as-quenched martensite) wasmeasured as follows. The steel plate was subjected to Repera corrosion.Then, the area ratio of a white contrast as the mixed structure of theas-quenched martensite and the retained austenite was measured by SEMobservation. The retained austenite fraction obtained by X-raydiffractometry was subtracted from the area ratio of the white contrast,thereby measuring the as-quenched martensite fraction.

The measurement results of the metal structures in each region of themolded article are shown in Table 3 below, and the mechanicalcharacteristics in each region of the molded article are shown in Table4 below.

TABLE 3 Structures of Molded Article (area %) First Molding RegionSecond Molding Region Test Steel Retained Other Bainitic As-QuenchedRetained Other No. Grade Martensite Austenite Structure FerriteMartensite Austenite Structure 1 A 95 5 0 0 95 5 0 2 A 95 5 0 85 7 8 0 3A 95 5 0 90 8 2 0 4 A 95 5 0 87 4 9 0 5 A 95 5 0 40 30 10 20 (Ferrite) 6B 100 0 0 95 5 0 0

TABLE 4 Mechanical Characteristics First Molding Region Second MoldingRegion Tensile Tensile Test Steel Strength Elongation StrengthElongation No. Grade TS (MPa) EL (%) TS (MPa) EL (%) 1 A 1550 10 1515 102 A 1550 10 1203 17 3 A 1550 10 1220 13 4 A 1550 10 1198 18 5 A 1550 10980 10 6 B 1545 7 1098 13

The following consideration was made based on these results. Test Nos. 2and 4 were examples satisfying the requirements defined in the presentinvention, and the results show that in each of Test Nos. 2 and 4, themolded article exhibiting high performance, i.e., a highstrength-ductility balance, in each region was obtained.

On the other hand, Test Nos. 1, 3, 5, and 6 were comparative examplesnot satisfying any of the requirements defined in the present invention,and any of the characteristics was lowered. That is, in Test No. 1, thestay time at (Bs—100° C.) to the Ms point was short in the second steelplate region, the fraction of the bainitic ferrite in the structure ofthe second region of the molded article was low, the fraction of themartensite in the structure of the second region of the molded articlewas high, and only a low elongation (a low total elongation EL) wasobtained in the second region.

In Test No. 3, the cooling rate change temperature was proper in thesecond steel plate region, but the stay time at (Bs—100° C.) to the Mspoint was short. Although a proper fraction of the bainitic ferrite inthe structure of the second region of the molded article was ensured,the amount of retained austenite was small. Thus, only a low elongation(a low total elongation EL) was obtained in the second region.

In Test No. 5, the cooling rate change temperature was high in thesecond steel plate region. The ferrite was formed, and the amount ofbainitic ferrite was not ensured. Thus, only a low strength and a lowelongation (a low total elongation EL) were obtained in the secondregion. In Test No. 6, a Si content was small in a steel component.Thus, even if the cooling conditions were proper, the amount of retainedaustenite was not formed in any of the regions of the molded article,and only a low elongation (a low total elongation EL) was obtained (thestrength in the second region was also low).

INDUSTRIAL APPLICABILITY

The press molded article of the present invention includes the firstmolding region exhibiting the metal structure which contains 80-97 area% of the martensite and 3-20 area % of the retained austenite and whichhas the residual structure at 5 area % or less; and the second moldingregion exhibiting the metal structure which contains 70-97 area % of thebainitic ferrite, 27 area % or less of the martensite, and 3-20 area %of the retained austenite and which has the residual structure at 5 area% or less. As a result, at least regions corresponding respectively to ashock-resistant area and an energy-absorbing area can be, withoutapplication of welding, formed in a single molded article, and ahigh-level balance between high strength and elongation can be achievedaccording to each region.

EXPLANATION OF REFERENCE NUMERALS

-   1 Punch-   2 Die-   3 Blank Holder-   4 Steel Plate (Blank)

1. A hot press molded article formed by hot press molding of a thinsteel plate, comprising: a first molding region exhibiting a metalstructure which contains 80-97 area % of martensite and 3-20 area % ofretained austenite and which has a residual structure at 5 area % orless; and a second molding region exhibiting a metal structure whichcontains 70-97 area % of bainitic ferrite, 27 area % or less ofmartensite, and 3-20 area % of retained austenite and which has aresidual structure at 5 area % or less.
 2. The hot press molded articleaccording to claim 1, wherein the first and second molding regions havean identical chemical component composition, and steel of each componentregion contains, in units of mass %, 0.15-0.3% of C, 0.5-3% of Si,0.5-2% of Mn, 0.05% or less of P, 0.05% or less of S, 0.01-0.1% of Al,0.01-1% of Cr, 0.0002-0.01% of B, [N]×4-0.1% of Ti, and 0.001-0.01% ofN, where 0% is not inclusive for the P and the S, and [N] denotes an Ncontent in units of %, and the steel of each component region has aresidual consisting of iron and an inevitable impurity.
 3. The hot pressmolded article according to claim 2, wherein the steel further contains,as other element, one or more selected from a group consisting of Cu,Ni, and Mo in a total amount of 1% or less, where 0% is not inclusive.4. The hot press molded article according to claim 2, wherein the steelfurther contains, as other element, at least one of V or Nb in a totalamount of 0.1% or less, where 0% is not inclusive.
 5. A method forproducing the hot press molded article according to claim 1 by forming athin steel plate so as to divide the thin steel plate into a pluralityof regions including at least first and second molding regions, themethod comprising: after the thin steel plate is heated to a temperatureof an Ac3 transformation point or higher and 1000° C. or lower, startingcooling at an average cooling rate of 20° C./sec or higher and moldingby pressing at least the first and second molding regions together witha die; and terminating, in the first molding region, the molding atequal to or lower than a temperature lower than a martensitetransformation start temperature by 50° C., and performing, in thesecond molding region, the cooling to a temperature range of equal to orlower than a temperature lower than a bainite transformation starttemperature by 100° C. and equal to or higher than the martensitetransformation start temperature and terminating the molding after alapse of a stay time of 10 seconds or longer within the temperaturerange.